Safety device for precision rotary location of crank fixtures in a machine tool

ABSTRACT

A latch blocks the opening of workpiece clamping arms in a machine tool, such as a crankpin grinding machine, until the crank fixtures are precisely angularly located. As the latch releases the clamping arms, a plunger is urged into an aperture in each crankhead positively locking the crank fixtures to the crankhead in the proper angular position for loading and unloading workpieces. Movement of the plunger into the crankhead closes a switch completing a circuit to open the clamping arms.

I United States Patent [151 3,646,710

Flohr, Jr. et a1. Mar. 7, 1972 [54] SAFETY DEVICE FOR PRECISION 2,651,895 9/1953 Rocks ..5l/237 CS X ROTARY LOCATION OF CRANK 2,926,471 3/1960 Bricker..... ....51/105 SP X L 3,118,258 1/1964 Price et al ..5l/105 SP FIXTURES IN A MACHINE T00 3,482,361 12/1969 Price et ...51/165.9 [72] Inventors: William P. Flohr, Jr., Waynesboro, Pa.; 2,807,974 10/1957 Meyer et a1 ..82/9

John A. Boiler, Thurmont, Md. Primary Examiner-Lester M. Swingle [73] Assignee. Landis Tool Company Anomey David S- y [22] Filed: Mar. 25, 1970 21 Appl. No.: 22,614 [57] ABSIRACT A latch blocks the opening of workpiece clamping arms in a machine tool, such as a crankpin grinding machine, until the (CL 51/105 S15?3 crank fixtures are precisely angularly located AS the latch releases the clamping arms, a plunger is urged into an aperture [58] field of Search ..51/105 R, 105 SP, 105 EC, 237 CS in each crankhead positively locking the crank fixtures to the crankhead in the proper angular position for loading and un- [56] cued loading workpieces. Movement of the plunger into the crank- UNITED STATES PATENTS head closes a switch completing a circuit to open the clamping arms. 1,755,937 4/1930 Steiner ..5 H237 CS 2,455,002 11/1948 Foudy ..5 H237 CS 18 Claims, 3 Drawing Figures R. H CRANKHEAD R,H. CRANKHEAD PATENTEDMAR 7 I972 SHEET 1 [1F 3 INVENTORS.

(I WlLLiAM P FLOHR, JRv

JOHN A BOLLER Quiz/#6 I ATTONEY locked within the bore 32. The piston 33 is urged against the inner surface of the bushing 36 by one or more compression springs 37 and 38. The latch member 11 is reduced in diameter at its end 39, and projects beyond the surface 41 of the crank fixture 18 into the path of the stop member 28 to limit the opening of the clamping arm 12.

In order to precisely position the crank fixtures 18-18 and prevent movement thereof during a loading and unloading operation, another latch in the form a plunger 42 is provided. The plunger 42 is slidably mounted in an axial bore 43 within each of the crank fixtures 18-18. The plunger 42 has a reduced diameter end portion 44 which projects through a spacer 46 and a seal 47, which are seated within an axial bore 48 in the spindle flange portion 26.

The plunger 42 is connected to the piston 33 by a pin 51 carried in a bore 52 in the plunger 42. The bore 52 is normal to the axis of the plunger 42 to enable the pin 51 to be anchored to the piston 33.

Rearward movement of the piston 33 (to the left in FIG. 2) advances the end portion 44 of the plunger 42 into a bushing 53 housed within an axial bore 54 formed in the flange portion 56 of the crankhead 19. The bushing 53 is retained against movement by a keeper 58 which is secured to the crankhead 19 as shown in FIG. 2.

Rearward movement of the plunger 42 is effected by the application of fluid pressure to the rod end of the cylinder 31. The latch member 11 is thus shifted inwardly to the left (FIG. 2) into its retracted position wherein the end portion 39 no longer blocks the clamping arm 12. The clamping arm 12 can now be opened by spring pressure when a signal is generated at the completion of the grinding cycle and the end portion 44 of the plunger 42 has been retracted into an internal bore 61 of the bushing 53. The bore 61 includes a tapered opening 62 which enables the end portion 44 of the plunger 42 to be guided axially against a switch plunger 63 which is slidably mounted in the bore 54. The plunger 63 includes a bore 64, housing a compression spring 66, which urges a shoulder 67 of the plunger 63 against the bushing 53.

The plunger 63 carries abutments 69-69 to effect the opening and closing of the limit switch LS. The closing of the limit switch 5LS provides a signal which allows the clamping arm 12 to be opened when the crank fixture 18 is positioned in the desired precise angular location.

Thus, the safety devices -10 ensure that the clamping arms 12-12 are not opened until the crank fixtures 18-18 and the crankheads 19-19 are in the proper angular position for loading and unloading. Also, the crank fixtures 18-18 are positively locked to the crankheads 19-19 to prevent any inadvertent angular displacement during the exchange of workpieces.

OPERATION With reference to FIGS. 1 and 2, it will be understood that the crankshaft 17 may be automatically loaded and unloaded from the grinding machine by a transfer apparatus such as loader 185, disclosed in the previously mentioned US. Pat. No. 3,118,258. The carriage 51 disclosed therein must be positioned longitudinally to align the crankpin in operable relation to the position of the grinding wheel 22.

The R.H. and L.H. crank fixtures 18-18 must be angularly positioned in the first or no. 1 station for grinding the correct crankpin, The safety devices 10-10 within the crank fixtures 18-18 will be actuated simultaneously to retract the latch members 11-11 enabling the clamping arms 12-12 to be opened fully before the crankshaft 17 can be placed in the machine manually or by the automatic loader.

After the unlatching occurs, the clamping arms 12-12 are opened by depressing the pushbutton selector switch PSS4 (FIG. 3) in the UNCLAMP position. Switch member PSS4A is opened and switch members PSS4A1 and PSS4C are closed. A circuit is completed through PSS4A] to energize an unlatch relay 19CR through contacts ICRHI, 70CR1, 39CR1, and a normally closed contact 31CR1.

The contact lCRHl was previously closed when the machine was set in the manual cycle, and the contact 70CR1 was closed when the crankshaft 17 was angularly positioned for grinding thereof in the first or No. 1 station.

The contact 39CR1 was closed by the energization of relay 39CR, which indicates that rotation of the crank fixtures 18- 18 was stopped in the proper angular position. A limit switch 19LS must be closed indicating the carriage 71 is positioned axially along a bed 75 to position the crankshaft 17 at the No. 1 station; normally closed contact 31CR2 is closed when the crank fixtures 18-18 are stopped, normally closed contact 40CR1 is closed when the wheelhead 25 has been retracted; and contact 34CR1 is closed when the sensing relay 34CR is energized.

Relay 34CR is energized when the proximity limit switch llLS is closed by rotation of an operating sensor (not shown) on one of the crankheads 19-19. The contact 34CR1 is closed to energize the relay 39CR through the limit switch 19LS and the normally closed contacts 31CR2 and 40CR1. A normally closed contact 39CR4 is opened to stop rotation of the motor MTR5. Rotation of the motor MTRS will be stopped automatically if the safety contact 19CR3 is opened.

Contact 39CR1 closes to energize the relay 19CR. The energization of the unlatch relay l9CR closes a contact 19CR1 to provide a holding circuit for the relay 19CR, through a normally closed contact 18CR1.

Contact 19CR2 is also closed, which energizes an unlatch solenoid SHBSOL (FIG. 2), which shifts a control valve 72 to the right. Hydraulic pressure is directed by a pump 73 driven by a motor 74 to direct fluid from a reservoir or supply tank 76 to a pressure reducing valve 77 through lines 78 and 79. A line 81 directs hydraulic fluid from the valve 77 to the valve 72.

A line 82 branching from line 81 directs fluid to a control valve 83. A check valve 84 and a check valve 85 are included in the line 81 on either side of the line 82 to regulate the direction of fluid flow to the valves 72 and 83.

The valve 83 is shown in its normal position (FIG. 2) wherein it directs fluid pressure through a line 86a and 86b to a directional valve 87 and thence to the head end of the clamp cylinder 13 by a line 88.

The clamp piston 89 and link member 91 are retained in an advanced position (to the left in FIG. 2) which holds the clamping arm 12 in a closed position in a conventional manner.

With the solenoid SHBSOL energized, fluid pressure from the valve 72 is directed to the rod end of the latching cylinder 31 through lines 92a and 92b. The piston 33 is retracted to the left, until it reaches a stop, provided by the flange portion 26 of the spindle 24. The end portion 44 of the plunger 42 is urged into the internal bore 61 of the bushing 53. The switch plunger 63 is retracted, which closes the limit switch SLS in the L.H. crankhead 19. It is to be understood that a corresponding limit switch 6LS (FIG. 3) is closed in the R.H. crankhead.

The closing of the limit switches SLS and 6LS energizes a relay 21CR indicating the crank fixtures 18-18 are properly locked in position. A contact 21CR1 is thereby closed to energize a timed relay 4TR, which closes a contact 4TR1 after a dwell period, to enable the latch members 11-11 to be retracted before the clamping arms 12-12 are unclamped.

The closing of contact 4TR1 energizes an unclamp relay 23CR through the normally closed contact 18CR2 and a contact 39CR3, which was closed by energization of relay 39CR when the crank fixtures 18-18 were located in the correct angular position for the first station, as shown in FIG. 1.

A contact 23CR1 is closed by the energization of the relay 23CR and the unclamp solenoid 6HBSOL is energized. The valve 83 is shifted to the left against spring pressure and a limit switch 7LS is closed. Fluid pressure from line 82 is blocked by movement of the valve 83. Therefore, fluid pressure which had been directed to the head end of the clamp cylinder 13 is blocked.

Pressure from a compression spring 93 retracts the piston 89 and the clamping arms 12- 12 are pivoted about a shaft 94 to their open position (not shown).

SAFETY DEVICE.FORPRECISION'ROTARY LOCATION OF CRANKFIXTURES IN A MACHINE TOOL BACKGROUND OF THE INVENTION This invention relates in-general to a safety device which positively prevents the opening of clamping arms holding a workpiece untila signalindicates the crank fixtures have been located in a preciseangular position. V

Prior to-this invention, rotation of the crank fixtures has been stopped in amapproximate position bymeans of a proximity switch located on'one of the crankheads. An additional proximity switch was used on the same crankhead to provide a signal to effect the'opening ofeach clamping arm when the crank fixture was correctly positioned angularly prior to loading-or unloading a crankshaft. While this approachhas been satisfactory, it is desired to maintain a positive angular positioning ofthe crank fixtures, particularly when the workpieces are to be loaded and unloaded automatically. Furthermore, it

is desired to provide a method oflocking the crank fixtures to g the crankheads in the precise angularposition to prevent inadvertent movement of the fixtures during loading and unloading.

SUMMARY OF THE INVENTION In accordance with the invention, the device includes a latch and plunger which are movable into position at the end of a machining cycle for locking a crank fixture to the headstock of a machine tool to positively angularly position the crank fixture. Facilities are rendered effective by the movement of the latch which retracts the plunger into a locking position and effects the opening of a clamping device on the crank fixture so that the workpiece canbe removed and another inserted.

In the preferred embodiment the device forming the subject invention is embodied in a crankpin grinding machine. The

latch provides a surface against which a stop member of the clamping device acts to block the complete opening of the clamping arm during rotary indexing and until the crank fixture is stopped in a predetermined angular position.

The plunger and latch are secured together to move in unison with one another. The plunger which is retracted by movement of the latch, includes a portion which enters an aperture on the headstock to lock the crank fixture to the headstock. Movement of the plunger into locking position renders operative an electric circuit which thereby disables a hydraulic holding device to permit spring opening of the clamping device.

Accordingly, it is a primary object of this invention to provide means for preventing the opening of clamping arms until.

a positive signal from each crankhead is obtained after each crank fixture has been precisely located angularly.

Another object is to provide means to prevent the clamping armsfrom being fully retracted until the crank fixtures are properly positioned and stopped.

Another object is to provide means for adjusting the clearance on each clampingarm to enable the crankshafts to be automatically indexed for the various pin stations.

Another object is to provide means to ensure that each crank fixture is in,proper angular alignment and in an operable relation to each other prior to loading or unloading a workpiece.

Another object is to provide means for locking the crank fixtures against rotation until the clamping arms are completely closed which mechanically permits the latch member to be advanced and thereby permits the plunger to be withdrawn from the aperture within the crankhead.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a partial rear view of an automatic crankpin grinding machine embodying the present invention and showing a safety device for locking the angular position of the crank fixtures;

FIG. 2 is a schematic diagram showing the hydraulic circuit for the safety device; and

'FIG. 3 is a diagram showing the electrical circuit for the safety device.

DESCRIPTIONOF THE PREFERREDEMBODIMENT crankhead is constructed in a like manner. While'the invention will be described with respect toan automatic crankpin grinding machine, it is to be understood that this is by way of example and not a limitation and that the invention has applicationv to machine tools in general.

Axial retraction of a latch member 11 is required before a clamping arm 12, which holds theworkpiece or crankshaft 17 in place, can be opened by spring pressure when fluid pressure to a clamp cylinder 13 is blocked. The retraction of the latch member 1 1 is initiated by a proximity switch llLS or switches which sense that the crank fixtures 18-18 are stopped in the proper angular position. Further details of the-crankheads as well as the overall grinding machine are disclosed in Price et al., U.S. Pat. NO. 3,] 18,258, issued on Jan. 21, l964 which is incorporated herein.

In a crankpin grinding machine, the outer bearing diameters 16-16 of the crankshaft 17, are secured by the clamping arms 12-12, which are pivotably mounted on the R.H. and L.H. crank fixtures 18-18. Each crank fixture 18-18 is rotatably mounted on the corresponding R.H. and L.H. headstock or crankhead 19-19 in a conventional manner.

The crankshaft 17 is rotatably supported as shown in FIG. 1, and is longitudinally positioned to align a crankpin 21 to be ground with a grinding wheel 22. The relative longitudinal position of the crankshaft l7 and the grinding wheel 22 is controlled by a reciprocating hydraulic motor M1. The grinding wheel 22 is advanced by the transverse movement of a wheel support or wheelhead 25 which is driven by a suitable electrical motor M2. The infeed rate may be varied by hydraulic and mechanical means to provide a rapid approach rate, a slow grinding feed rate, and a fine feed rate, until size is indicated by a gauging device. The infeed of the grinding wheel 22 is controlled by a suitable electrical motor M3. Such a feed mechanism is disclosed in Price et al., U.S. Pat. No. 3,482,361, issued Dec. 9, I969.

The axis of the work drive spindles 24-24 is in line with the axis of one of the crankpins 21 or 30 during the grinding operation. Each of the spindles 24-24 has a flange portion 26, to which the crank fixtures 18-18 are attached. Each of the fixtures 18-18 includes the clamping cylinder 13 and the pivotably mounted clamping arm 12. The crankshaft 17 is angularly positioned by facilities (not shown) in order to place the eccentric pin 21 or 30 to be ground coaxially with the drive spindles 24-24.

The clamping arms 12-12 secure the outer bearing diameters 16-16 of the crankshaft 17 against conventional bearing blocks 27-27 during the grinding operation. The clamping arms 12-12 are adjusted to provide a minimum amount of clearance through an. adjustable stop member 28 (FIG. 2) which is in threaded engagement with a threaded bore 29 within each of the clamping anns 12-12. Adjustment of the stop member 28 varies the amount that the clamping arm 12 may be retracted, while the crankshaft 17 is being angularly repositioned or indexed in preparation forgrinding a different crankpin. The clamping arms 12-12 should permit rotational movement of the crankshaft during indexing, but should not open so far that the crankshaft might be inadvertently released.

A latch cylinder 31, as shown in FIG. 1 and FIG. 2, is formed in axial bore 32 within each crank fixture 18-18 and includes a piston 33. The latch member 11, which is an extension of the piston 33, is slidably journaled in a bushing 36 Fluid pressure is directed from the head end of the clamp cylinder 13 by line 88 through the valve 87. The fluid is drained back to the tank 76 through line 96 as the pilot member 97 is raised by spring pressure when the valve 83 is shifted to the left.

Limit switches 8LS and 9LS are closed when the clamping arms 12-12 are fully unclamped, to energize a clamps open relay 27CR, through the limit switch 7LS, which was previously closed when valve 83 was shifted.

The energization of the relay 27CR closes a contact (not shown), which energizes a load-unload relay (not shown), to enable the loading of an unground crankshaft 17 into the machine during the automatic cycle.

The automatic cycle of the machine can now be effected by pushbuttons (not shown), which close the automatic relay contacts CRAl, CRA2 and CRA3. An unground crankshaft 17 will be automatically loaded into the crank fixtures 1818 by a conventional loader.

A limit switch 4L5 is closed when the crankshaft 17 is properly positioned longitudinally by a work locator (not shown). The closing of the limit switch 4LS energizes a timed relay 3TR through the contact CRAl and contact 39CR2. The contact CRAl was closed at the start of the automatic cycle, and the contact 39CR2 was closed when the crank fixtures 1818 were angularly positioned for the first station, thereby energizing relay 39CR.

Energization of the relay 3TR closes the contact 3TR1 which completes a circuit to clamp relay 18CR, through the contact CRA2 and a normally closed contact 31CR3. The energization of the relay 18CR opens normally closed contacts 18CR1, 18CR2 and 18CR5. Contacts 18CR3 and 18CR4 are thereby closed. Closing of the contact 18CR3 provides a holding circuit around the contact 3TR1 which will be opened later in the cycle; and the contact 18CR4 provides a holding circuit for the relay 18CR during the manual cycle through the manual contact lCRHZ.

Opening of the normally closed contact 18CR2 deenergizes the relay 23CR which opens the contact 23CR1 and deenergizes the solenoid 6HBSOL. The valve 83 is then shifted to the right (H6. 2) to its normal position by spring pressure, and the limit switch 7LS is opened. Fluid pressure is directed through the valve 87 by lines 860 and 86b, and to the head end of the clamp cylinders 13 through the line 88.

The piston 89 and the link member 91 are advanced by fluid pressure to effect the closing of the clamping arm 12 which is pivoted about shaft 94 until the bearing diameters 16l6 of the crankshaft 17 are secured against the bearing blocks 27- 27.

The limit switches 8LS and 9LS are opened or reset when the clamping arms 12-12 are closed, which deenergizes the clamps open relay 27CR.

The unlatch relay 19CR was deenergized by the opening of the normally closed contact 18CR5. The contact 19CR2 was thereby opened which decnergized the solenoid SHBSOL. The valve 72 is shifted to the left (FIG. 2) by spring pressure, and fluid pressure which had been directed to the rod end of the latch cylinder 31 is blocked.

Spring pressure within the latch cylinder 31 shifts the piston 33 so that the end portion 39 of the latch 11 is in the advanced or latched position and fluid pressure is directed by lines 92b and 92a to the valve 72 and returned to the tank 76 by a line 98. The clamping arms 12-12 cannot be retracted fully until all ofthe crankpins 2l2l and 3030 have been ground.

It is to be understood that the adjustable stop members 28- 28 are locked in each of the clamping arms l2-12 to enable a minimum amount of clearance between the latch 11-l1 prior to the grinding operation. This clearance will enable the clamping arms 12 12 to be partially opened or retracted during the indexing ofthe crankshaft 17 while the crankshaft 17 is being axially positioned along the bed 75 from one carriage station to the next.

Positioning ofthe carriage 71 is required when grinding successive crankpins 212l having identical dimensions from the axis of the main bearing diameters 16-16 in a conventional manner.

The crank fixtures 18l8 are angularly positioned at a preset rate by a reciprocating motor (not shown), which transfers movement through an over-running clutch, in a conventional manner. Angular movement of the crank fixtures 18- 18 continues until a signal from a proximity switch 11LS closes and stops rotation of the crank fixtures l8-18 in the desired angular position.

Unlatching occurs as described above and the clamping arms 12-12 are opened to allow the finish ground crankshaft 17 to be removed from the machine by the automatic loader.

An unground crankshaft 17 will be loaded into the machine and the automatic cycle will be repeated. The clamping arms l2, 12 may be closed in the manual cycle by depressing the pushbutton selector switch P584 in the CLAMP position. The switch member PSS4B is closed which energizes the relay 18CR through the contact 19CR4 and the contact 1CRH2. The contact 19CR4 is closed when the unlatch relay 19CR is energized, and the contact 1CRH2 is closed as discussed in the automatic cycle.

While the apparatus disclosed herein is a safety device for the purpose of locking the angular position of the crank fixtures relative to the RH. and L.H. headstocks, the invention also prevents the loading or unloading of a crankshaft during the manual or automatic cycle of the machine until the crank fixtures are properly positioned.

It is to be noted that rotation of the crank fixture 18 is prevented until the clamping am 12 is completely closed. This results from the fact that the end portion 39 of the latch member 11 would abut against the side of the clamping arm 12 if the latch member 11 were advanced prior to the closing of the clamping arm 12, and the end portion 44 of the plunger 42 would still be engaged in the bore 61.

lt is to be understood that the invention is not limited to the particular embodiments and features described and shown, but that it comprises any modifications and equivalents within the scope of the appended claims.

We claim:

1. In a machine tool for removing stock from a workpiece, the machine too] including a tool, a pair of headstocks each having a spindle, a fixture including a clamping device rotatably mounted on said spindle for holding a workpiece, means for rotating said spindles and fixtures, wherein the improvement comprises:

a plunger movable into position at the end of a machining cycle for locking said fixture to said headstock in a precise angular position; and

means rendered effective by the locking movement of said plunger for opening said clamping device to enable a workpiece to be loaded or unloaded into the machine tool.

2. A machine tool as recited in claim 1, which includes, a retractable latch operating in unison with said plunger before a machining operation for limiting the extent of opening of said clamping device and to permit rotation of said clamping device.

3. A machine tool as recited in claim 1, which further comprises:

a latch connected to said plunger and movable in unison therewith to limit the opening of said clamping device, and

means for retracting said latch when said fixture stops in a predetermined angular position.

4. A machine tool as recited in claim 3, wherein clearance is provided between the latch and the clamping device so that the fixture can be rotated relative to the workpiece during the machining cycle for machining different surfaces of the workpiece.

5. A machine tool as recited in claim 1, which further comprises:

a latch connected to said plunger and movable in unison therewith to limit the opening of said clamping device, and

means for advancing said plunger into position to lock the fixture against rotation to enable a workpiece to be loaded or unloaded into the machine tool.

6. A machine tool as recited in claim 1, wherein said opening means includes:

an electric switch actuated by said plunger;

hydraulic means for holding said clamping device in a closed position, said hydraulic means being rendered inoperative by actuation of said electric switch; and

a spring for opening said clamping device when said hydraulic means is inoperative.

7. A machine tool as recited in claim 1, wherein, at least one of said headstocks includes means coacting with said plunger to lock the fixture in a predetermined angular position.

8. In a machine tool for removing stock from a workpiece, the invention comprising:

at least one headstock;

at least one fixture supported by the headstock for holding the workpiece, said fixture being mounted for rotation relative to the headstock during a machining cycle;

means for rotating said fixture during a machining cycle;

a latch for limiting the opening of said fixture, said latch being retracted into a nonblocking position when said fixture stops in a predetermined angular position; and

plunger connected to said latch, said plunger providing a positive lock between the fixture and the headstock to precisely position the fixture with respect to the headstock and prevent relative rotation therebetween when the latch is retracted into its inoperative position.

9. A machine tool as recited in claim 8, which further comprises:

means rendered effective by said plunger assuming its locking position for opening said fixture to release the workpiece.

10. A machine tool as recited in claim 8, wherein said opening means includes:

an electric switch actuated by said plunger,

hydraulic means for holding said fixture in a closed position,

said hydraulic means being rendered inoperative by actuation of said electric switch; and

a spring for opening said fixture when said hydraulic means is inoperative.

11. A machine tool as recited in claim 8, wherein a clearance between said latch and the fixture is provided so that the fixture can be rotated relative to the workpiece during the machining cycle for machining different surfaces of the workpiece.

12. A machine tool as recited in claim 8, wherein said headstock includes an axially directed aperture, and said plunger is movable axially into said aperture to lock the fixture to the headstock.

13. A machine tool as recited in claim 8, wherein said fixture includes a clamping device, said device being positioned in the path of said latch so that the latch and plunger cannot be advanced from their retracted position until said device is closed.

14. In a machine for grinding crankshafts, the machine including a grinding wheel, means for supporting and rotating said wheel. a pair of headstocks each having a spindle, a crank fixture including a clamping device rotatably mounted on each spindle for holding a crankshaft eccentrically, means for rotating said spindles and crank fixtures, means for rotating the crankshaft relative to said spindles and fixtures to align a crankpin with the axis of spindle rotation, means for effecting relative longitudinal and transverse movements between said crankshaft and said grinding wheel, wherein the improvement comprises:

a plunger movable into position at the end of a machining cycle for locking said crank fixtures to said headstocks to precisely angularly position said crank fixtures; and

means rendered effective by the locking movement of said plunger for opening said clamping devices to enable a crankshaft to be loaded or unloaded into the machine tool.

15. In a machine for grinding crankshafts, the machine including a grinding wheel, a pair of headstocks each having a spindle, a crank fixture including a clamping device for holdmg a crankshaft eccentrically, means for rotating said spindles and crank fixtures, wherein the improvement comprises:

a latch for physically blocking the opening of said clamping device until said crank fixture is in a predetermined angular position;

means for retracting said latch from its blocking position into a nonblocking position;

a plunger supported on said crank fixture and movable in unison with said latch into locking engagement with said headstock to lock said crank fixture in a precise angular position relative to said headstock; and

means actuated by said plunger once it is in locking engagement with said headstock for opening said clamping device so that said crankshaft can be removed and another inserted.

16. A crankpin grinding machine as recited in wherein said opening means comprises:

an electric switch actuated by said plunger;

hydraulic means for holding said crank fixtures in a closed position, said hydraulic means being rendered inoperative by actuation of said electric switch; and

a spring for opening said fixture when said hydraulic means is inoperative.

17. A crankpin grinding machine as recited in claim 15 wherein clearance is provided between the latch and crank fixture so that the crankshaft can be rotated relative to the crank fixture during the grinding cycle for grinding different pins of the crankshaft.

18. A crankpin grinding machine as recited in claim 15 wherein said clamping device includes:

a pivotally mounted clamping arm; and

a stop member secured to the clamping arm, said stop member being in abutting relationship to said latch to prevent opening of said arm;

said stop member being adjustably mounted on said arm to provide a clearance between said stop member and said latch so that said arm can be opened a sufficient distance for angular indexing of said crankshaft.

claim 15 

1. In a machine tool for removing stock from a workpiece, the machine tool including a tool, a pair of headstocks each having a spindle, a fixture including a clamping device rotatably mounted on said spindle for holding a workpiece, means for rotating said spindles and fixtures, wherein the improvement comprises: a plunger movable into position at the end of a machining cycle for locking said fixture to said headstock in a precise angular position; and means rendered effective by the locking movement of said plunger for opening said clamping device to enable a workpiece to be loaded or unloaded into the machine tool.
 2. A machine tool as recited in claim 1, which includes, a retractable latch operating in unison with said plunger before a machining operation for limiting the extent of opening of said clamping device and to permit rotation of said clamping device.
 3. A machine tool as recited in claim 1, which further comprises: a latch connected to said plunger and movable in unison therewith to limit the opening of said clamping device, and means for retracting said latch when said fixture stops in a predetermined angular position.
 4. A machine tool as recited in claim 3, wherein clearance is provided between the latch and the clamping device so that the fixture can be rotated relative to the workpiece during the machining cycle for machining different surfaces of the workpiece.
 5. A machine tool as recited in claim 1, which further comprises: a latch connected to said plunger and movable in unison therewith to limit the opening of said clamping device, and means for advancing said plunger into position to lock the fixture against rotation to enable a workpiece to be loaded or unloaded into the machine tool.
 6. A machine tool as recited in claim 1, wherein said opening means includes: an electric switch actuated by said plunger; hydraulic means for holding said clamping device in a closed position, said hydraulic means being rendered inoperative by actuation of said electric switch; and a spring for opening said clamping device when said hydraulic means is inoperative.
 7. A machine tool as recited in claim 1, wherein, at least one of said headstocks includes means coacting with said plunger to lock the fixture in a predetermined angular position.
 8. In a machine tool for removing stock from a workpiece, the invention comprising: at least one headstock; at least one fixture supported by the headstock for holding the workpiece, said fixture being mounted for rotation relative to the headstock during a machining cycle; means for rotating said fixture during a machining cycle; a latch for limiting the opening of said fixture, said latch being retracted into a nonblocking position when said fixture stops in a predetermined angular position; and plunger connected to said latch, said plunger providing a positive lock between the fixture and the headstock to precisely position the fixture with respect to the headstock and prevent relative rotation therebetween when the latch is retracted into its inoperative position.
 9. A machine tool as recited in claim 8, which further comprises: means rendered effective by said plunger assuming its locking position for opening said fixture to release the workpiece.
 10. A machine tool as recited in claim 8, wherein said opening means includes: an electric switch actuated by said plunger, hydraulic means for holding said fixture in a closed position, said hydraulic means being rendered inoperative by actuation of said electric switch; and a spring for opening said fixture when said hydraulic means is inoperative.
 11. A machine tool as recited in claim 8, wherein a clearance between said latch and the fixture is provided so that the fixture can be rotated relative to the workpiece during the machining cycle for machining different surfaces of the workpiece.
 12. A machine tool as recited in claim 8, wherein said headstock includes an axially directed aperture, and said plunger is movable axially into said aperture to lock the fixture to the headstock.
 13. A machine tool as recited in claim 8, wherein said fixture includes a clamping device, said device being positioned in the path of said latch so that the latch and plunger cannot be advanced from their retracted position until said device is closed.
 14. In a machine for grinding crankshafts, the machine including a grinding wheel, means for supporting and rotating said wheel, a pair of headstocks each having a spindle, a crank fixture including a clamping device rotatably mounted on each spindle for holding a crankshaft eccentrically, means for rotating said spindles and crank fixtures, means for rotating the crankshaft relative to said spindles and fixtures to align a crankpin with the axis of spindle rotation, means for effecting relative longitudinal and transverse movements between said crankshaft and said grinding wheel, wherein the improvement comprises: a plunger movable into position at the end of a machining cycle for locking said crank fixtures to said headstocks to precisely angularly position said crank fixtures; and means rendered effective by the locking movement of said plunger for opening said clamping devices to enable a crankshaft to be loaded or unloaded into the machine tool.
 15. In a machine for grinding crankshafts, the machine including a grinding wheel, a pair of headstocks each having a spindle, a crank fixture including a clamping device for holding a crankshaft eccentrically, means for rotating said spindles and crank fixtures, wherein the improvement comprises: a latch for physically blocking the opening of said clamping device until said crank fixture is in a predetermined angular position; means for retracting said latch from its blocking position into a nonblocking position; a plunger supported on said crank fixture and movable in unison with said latch into locking engagement with said headstock to lock said crank fixture in a precise angular position relative to said headstock; and means actuated by said plunger once it is in locking engagement with said headstock for opening said clamping device so that said crankshaft can be removed and another inserted.
 16. A crankpin grinding machine as recited in claim 15 wherein said opening means comprises: an electric switch actuated by said plunger; hydraulic means for holding said crank fixtures in a closed position, said hydraulic means being rendered inoperative by actuation of said electric switch; and a spring for opening said fixture when said hydraulic means is inoperative.
 17. A crankpin grinding machine as recited in claim 15 wherein clearance is provided between the latch and crank fixture so that the crankshaft can be rotated relative to the crank fixture during the grinding cycle for grinding different pins of the crankshaft.
 18. A crankpin grinding machine as recited in claim 15 wherein said clamping device includes: a pivotally mounted clamping arm; and a stop member secured to the clamping arm, said stop member being in abutting relationship to said latch to prevent opening of said arm; said stop member being adjustably mounted on said arm to provide a clearance between said stop member and said latch so that said arm can be opened a sufficient distance for angular indexing of said crankshaft. 